A sublimator is a primary cooling device for the Extravehicular Mobility Unit (EMU) Space Suit and for the Orion Spacecraft. Its functional heart is a metallic porous plate that is exposed to space vacuum on one side. It is supplied with expendable feed-water on the other side. In operation, the feed-water freezes on the porous plate surface, and the vacuum side progressively sublimes water from this ice to the vacuum of space as waste heat is introduced into the plate.
A sublimator has two fluid loops inside. Both loops are closed. One loop takes fluid from a heat source and the other takes fluid to the porous plate surface. There is no mixing of fluids and the sublimation loop takes heat by metal to metal contact.
Experience has shown that trace contaminants in feed-water (primarily amphipathic long-chain organic acids, fatty acids and surfactants with carboxylate functional groups form a sublimation impeding molecular monolayer that can drastically impact the sublimation process and therefore sublimator performance. Trace processing aids, as exemplified by abietic acid from the early EMU Neoprene Latex feed-water bladders, sodium dodecyl benzene sulfonate (a candidate soap for International Space Station-ISS processed water), and acrylic acid oligomers from one of the current operational ISS Water Processor multi-filtration bed sorbents have exhibited this phenomena.
It is believed that the hydrophilic “heads” of the amphipathic molecules anchor to available cationic charge on the metallic surface of the porous plate, and that the hydrophobic “tails” of the amphipathic molecules form an intertwined monolayer much the same as a lipid bilayer in a cell. This phenomenon does not occur with non-volatile contaminants that do not have the structural features of an amphipathic molecule such as corrosion products, iodine and biofilm. These contaminants merely dislodge from the effluent side of the porous plate during the sublimation process and have minimal effect on performance.
The sensitivity of a sublimator to trace amphipathic molecules that form monolayers (very common processing aids for non-metallic materials) severely restricts the types of materials that can be used to contain and/or transfer feed-water, and it restricts the sources of the feed-water. This has becomes a serious logistics challenge on long-term missions such as on the International Space Station where recycled water that contains trace amounts of acrylic acid oligomer from one of the Water Processor Assembly sorbents is the preferred source of EMU feedwater. What is needed in the art is a means to passivate a sublimator surface to the anchoring of amphipathic contaminants.